This invention relates to zone refining, sometimes called zone melting, and more specifically to a means for providing an extended interface between the molten zone and the adjacent solid material through the use of extended heat exchange surfaces. Zone refining makes use of the principle that crystallization of a homogeneous liquid mixture results in a solid whose composition is usually different from the composition of the liquid. Zone refining is a fractional solidification process wherein, for example, a molten zone is caused to move slowly through a solid charge from one end to the other for the purpose of purification of a portion of the solid material. Impurities are concentrated ahead of the freezing interface of the molten zone when the operating conditions are properly chosen and when the equilibrium concentration of each impurity is greater in the liquid than in the solid. The process has been widely used for the preparation of ultra-pure materials but it also is useful in the separation of less pure materials. Although several molten zones may be passed through a solid charge simultaneously, the process still suffers from low productivity, especially at higher impurity levels. When a molten zone is moved too rapidly through a solid charge, impurities are not effectively rejected and become trapped in the freezing liquid/solid interface. In general, zone refining devices are operated at a compromise between a good productivity rate and an acceptable rate of entrapment so as to provide the maximum output of material of a given purity per unit time and per unit volume of ingot.